1. Field of the Invention
The present invention relates to a droplet discharging apparatus, a method of controlling a droplet discharging apparatus and an image forming apparatus including a droplet discharging apparatus.
2. Description of the Related Art
An inkjet printer is widely known as an image forming apparatus in which ink is injected on a recording medium. Such an inkjet printer includes a recording head, and prints on a recording medium by discharging ink droplets from nozzles formed at the recording head toward the recording medium. Further, an inkjet printer is known that includes a unit capable of discharging a special color ink (white, for example) or a magnetic material ink in addition to general YMCK inks.
For ink used for such an inkjet printer (printer apparatus), a dye ink and a pigment ink are used. The pigment ink has a feature that it is brightly colored as the pigment ink uses pigment as colorant and the pigment is dispersed in ink solvent. However, there is a problem that the colorant sediments if the pigment ink is left for a long time.
Further, the pigment ink includes a white ink that is known to have strong tendency that pigment sediments because the specific gravity of titanium oxide, which is the pigment of the white ink, is large. Further, among ink kinds, a magnetic material ink is known that includes a magnetic material and is mainly used for printing bank checks or the like. Similarly, the magnetic material tends to sediment because the specific gravity of the magnetic material is large. Such inks in which segmentations tend to occur are referred to as “sedimentary inks”, hereinafter. If the sedimentary ink is left, physical property values (viscosity or the like) of the ink change because the ink cannot retain a normal condition in which the pigment is uniquely mixed. As a result, a printing failure occurs due to lowering of discharging characteristics, or lowering of image quality such as the concentration is not uniform occurs.
Thus, a technique is known in which remaining oscillation after supplying a drive signal to a piezoelectric element is detected to estimate ink viscosity, and a recovery process is performed (Patent Document 1). In this example, as illustrated in FIG. 22, an oscillation period Tc of the remaining oscillation is measured, and a period Ts from a timing when supplying of a drive signal to a piezoelectric element is finished to a timing when a remaining oscillation waveform exceeds a reference voltage Vref is detected. Then, a peak value Em of the remaining oscillation waveform of a first half-wave is obtained based on a ratio Ts/Tc of the oscillation period Tc and the period Ts, and the reference voltage Vref, and the viscosity of the ink is determined based on the obtained peak value Em.
Further, an inkjet printer is disclosed that includes an ink circulation path in which an inkjet head, a first tank that supplies ink to the inkjet head and a second tank that collects the ink that is not consumed at the inkjet head are provided. The inkjet printer further includes a maintenance unit that performs a maintenance operation accompanied with consumption of ink, a measurement unit that obtains a flow channel resistance when ink flows from the first tank to the second tank via the inkjet head, and a control unit that selects an maintenance operation by which the ink is circulated in the ink circulation path without accompanying the consumption of the ink by the maintenance unit (Patent Document 2).
However, according to the technique of Patent Document 1, as will be explained later, there is a problem that slight change of the viscosity of the ink cannot be detected because variation of ON resistor/ON time of a switching unit directly influences on the above described period Ts and the peak value Em of the first half-wave varies largely.
Further, according to the inkjet printer of Patent Document 2, as the flow channel resistance is obtained by measuring the period necessary for the ink to flow from the first tank to the second tank, it is necessary to drive a pump for circulating the ink after a predetermined period for obtaining the flow channel resistance has passed. Further, the pump is driven after the predetermined period has passed regardless of the viscosity of the ink. Thus, there is a problem that the pump is always periodically driven and power consumption is increased.